1. Field of the Invention
The invention relates to a radar system adapted for shipboard or airborne operation in which a gyro compass produces signals which are used for providing stabilization of the radar display so that the radar presentation upon the display screen does not rotate along with course changes of the ship, and for providing course related outputs such as visual readout of true bearing.
2. Description of the Prior Art
Shipboard radar systems customarily include a gyro compass which produces incremental heading information to the radar system for rotation of a true bearing scale, for providing north stabilization of the radar display, and for providing true motion correction. The most common gyro compass systems have either stepper motor or synchro-types of outputs. In the case of a stepper motor, three pole sets are commonly used with either 10' or 20' bearing change per output step. Pulse amplitudes of 35, 50 and 70 volts are normally employed. Synchro outputs, which are ordinarily three-phased, produce analog outputs representing either 1.degree. or 2.degree. of bearing per 360.degree. of shaft rotation. Somewhat less common are synchro outputs representing 10.degree. of bearing per 360.degree. of shaft rotation. The rotor is driven from a supply frequency of either 50, 60 or 400 Hz with commonly used voltages of 50, 60, 62, 115, 125 and 150 volts. The stator voltages are known to range between 20 and 90 volts with 20, 24, 57, 68, 82 and 90 volts being used in the most common commercially available units.
In past practice, the gyro compass course inputs were used to drive expensive mechanical devices, such as stepping motors, resolvers, and sine-cosine potentiometers to generate a stabilized and oriented PPI display and to rotate a true bearing scale.
Common electronic circuitry to drive the mechanical linkage from a DC stepper or 360:1 or 180:1 synchro-type gyro compass input, adaptable to a wide range of rotor and stator voltages have lately become available, avoiding the need for a new design for each different configuration. An example of one version of such a system is shown in U.S. Pat. No. 4,107,007, of J. E. Bryden, filed on Jan. 5, 1976, issued on Jan. 3, 1978, and assigned to the present assignee. However, compass systems with a synchro coupling, such, for example, that the required incremental step corresponds to a precision higher than a one sixth division of a full synchro revolution, usually requires a special mechanical solution such as remote duplication of the synchro shaft rotation to drive a version of a well-known optical shaft encoder and to acquire the desired subdivision. Such duplication adds to expense and system complexity.
Shipboard gyro compass output systems are usually susceptable to signal distortions such as high amplitude common mode transients and to uncertainties in transitions on the high level phase output gyro signal lines, resulting in display jitter, erroneous bearing read outs and, therefore, the need for frequent alignments.